Basically WordPress allows you to manipulate your configuration file so that when certain criterions are met you can override the default home and blog URI’s set in the database.

By doing so you can have more or less unlimited domains pulling the same files and data from one single WordPress installation.

Here’s how you do it

Open wp-config.php in your favourite editor (Notepad, Dreamweaver etc.) and add the following:

if (preg_match("/\bwww.example1.com\b/i", $_SERVER['REQUEST_URI']) || preg_match("/\bwww.example1.com\b/i", $_SERVER['HTTP_HOST'])) {
define('WP_SITEURL', 'http://www.example1.com');
define('WP_HOME', 'http://www.example1.com');
}

If you need more domains just add more by repeating the above starting the next “block” with elseif.

Feel free to comment and/or suggest different ways of doing this.